Retroreflective materials are characterized by the ability to redirect light incident on the material back toward the originating light source. This property has led to the widespread use of retroreflective sheeting for a variety of traffic and personal safety uses. Retroreflective sheeting is commonly employed in a variety of articles, for example, road signs, barricades, license plates, pavement markers and marking tape, as well as retroreflective tapes for vehicles and clothing.
Two known types of retroreflective sheeting are microsphere-based sheeting and cube corner sheeting. Microsphere-based sheeting, sometimes referred to as “beaded” sheeting, employs a multitude of microspheres typically at least partially embedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes or vapor coats, etc.) to retroreflect incident light. Due to the symmetrical geometry of beaded retroreflectors, microsphere based sheeting exhibits the same total light return regardless of orientation, i.e., when rotated about an axis normal to the surface of the sheeting. Thus, such microsphere-based sheeting has a relatively low sensitivity to the orientation at which the sheeting is placed on a surface. In general, however, such sheeting has a lower retroreflective efficiency than cube corner sheeting.
Cube corner retroreflective sheeting, sometimes referred to as “prismatic” sheeting, typically comprises a thin transparent layer having a substantially planar first surface and a second structured surface comprising a plurality of geometric structures, some or all of which include three reflective faces configured as a cube corner element.
Cube corner retroreflective sheeting is commonly produced by first manufacturing a master mold that has a structured surface, such structured surface corresponding either to the desired cube corner element geometry in the finished sheeting or to a negative (inverted) copy thereof, depending upon whether the finished sheeting is to have cube corner pyramids or cube corner cavities (or both). The mold is then replicated using any suitable technique such as conventional nickel electroforming to produce tooling for forming cube corner retroreflective sheeting by processes such as embossing, extruding, or cast-and-curing. U.S. Pat. No. 5,156,863 (Pricone et al.) provides an illustrative overview of a process for forming tooling used in the manufacture of cube corner retroreflective sheeting. Known methods for manufacturing the master mold include pin-bundling techniques, direct machining techniques, and techniques that employ laminae. Commercially available methods of producing microreplicated prismatic sheeting include, for example, U.S. Pat. No. 7,410,604 (Erickson), PCT Patent Publication No. 2007124217 (Thakkar), U.S. Pat. No. 6,200,399 (Thielman), and U.S. Pat. No. 5,691,846 (Benson). These microreplication processes produce a retroreflective sheeting with prismatic structures that have been precisely and faithfully replicated from a microstructured tool having a negative image of the desired prismatic structure.
Prismatic retroreflective sheeting is known for returning a large portion of the incident light towards the source (Smith, K. Driver-Focused Design of Retroreflective Sheeting For Traffic Signs, in Transportation Research Board 87th Annual Meeting: Compendium of Papers DVD, Washington D.C. 2008). Many commercially available products rely on the relatively high retroreflectance (light return toward the source) provided by prismatic cube corner microstructures to meet high retroreflectance specifications (e.g., retroreflectance (RA) or brightness in the range of 300 to 1000 candela per lux per meter square (cpl) for 0.2 degree observation angle and −4 entrance angle), such as ASTM types III, VII, VIII, IX, and X, as described in ASTM D 4956-04, and type XI.
However, prismatic cube corner microstructures have not been used in products designed to meet lower retroreflectance specifications (e.g., RA in the range of 70 to 250 cpl for 0.2 degree observation angle and −4 entrance angle for white sheeting), such as ASTM types I and II as described in ASTM D 4956-04. Instead, commercially available ASTM type I and II products utilize glass beads embedded in multiple layers of polymeric materials as the optical elements. A specular reflective coating, typically vacuum deposited aluminum, is situated behind the glass beads near the light focal point to enable retroreflection.